1. Field of the Invention
The invention relates more precisely to a very broadband receiver intended for instantaneously measuring the frequency of signals received (or intercepted) by the receiver notably in the field of radar frequencies.
2. Description of the Related Art
Receivers intended for detecting radioelectric signals of radar (or telecommunications) type are notably used in the field of electronic warfare. These types of receivers comprise functions for measuring instantaneous frequency and the level of the signal received.
The technical problem which arises for such receivers is that of measuring the frequency of the intercepted signals with:
good sensitivity; the noise power intrinsic to an instantaneous broadband receiver necessitates working with a very low, or even negative, signal-to-noise ratio to obtain the required sensitivity,
low duration of measurement, and therefore of integration; the aim being to characterize radar emissions whose pulse duration may be very short,
signal measurements intercepted at a high rate; the aim is to associate the measurements of frequency and level so as to characterize the signals intercepted with good temporal resolution,
a low-cost and low-volume measurement receiver; a significant improvement in this field makes it possible, for example in detection systems, to multiply the number of receivers per system and therefore to improve the performance of the system without weighing on either the price or the volume of the system.
State of the art receivers operating at very high frequencies generally comprise analog input stages followed by digital signal processing stages.
An operating principle of a state of the art receiver allowing instantaneous measurement of the frequency of the signal received designated by “frequency meter with spatial sampling” is based on the creation of a standing wave regime, on the basis of the signal received, in a propagation line and on measuring the position of the nodes and antinodes distributed along this propagation line, the periodicity of the nodes and antinodes gives a coarse measurement of the frequency of the signal received.
FIG. 1 represents a principle of a state of the art receiver showing a practical way of obtaining this standing wave regime.
A signal received se(t), after analog amplification, is distributed by a power distributor 10 to two outputs of the distributor. Each signal s1(t) and s2(t) at the two outputs of the distributor 10 is applied, one s1(t) to an input E1 of an analog propagation line L1 and the other s2(t) to an input E2 of another analog propagation line L2, the two analog lines being oriented along an axis XX′. The other analog line L2 comprises in series a delay line Lr. The two lines L1 and L2 are connected by their output S.
The signal s1(t) propagates on the line L1 in the increasing direction of x along this axis XX′. Another signal s2(t-τ) obtained on the basis of the signal s2(t) delayed by τ by the delay line Lr is injected by the line L2 at the other end S of the line L1 and propagates on the line L1 in the direction of decreasing x. This results in a standing wave regime Stw along the propagation line L1 producing voltage nodes and antinodes whose periodicity provides a coarse measurement of the frequency F of the signal received by the receiver.
The positions of the nodes and antinodes are detected by a row Rd of detecting diodes disposed along the line L1 whose output signals form the subject of an analog 20 and then digital 22 signal processing providing this coarse frequency measurement F of angular frequency ω.
The position of the nodes and antinodes is expressed by the expression
            g      ⁡              (        x        )              =          k      ·              A        2            ·              [                  1          +                      cos            ⁡                          [                                                2                  ⁢                  ω                  ⁢                                                                          ⁢                  x                                Vo                            ]                                +          φ                ]              with      f    =          φ              2        ⁢                  π          ·          τ                    
The position of the nodes and antinodes (therefore the term φ) gives a fine measurement of the frequency, but ambiguous.
g(x) is sampled along the propagation line L1. A discrete Fourier transform makes it possible to extract the coarse frequency measurement and the ambiguous fine measurement. The compromises of precision that are necessary for resolving the ambiguity must be made.
In another operating principle of the state of the art receiver termed “auto-correlator with delay line”, the idea is to directly measure the phase difference φ induced by a delay line 30 of delay τ.
FIG. 2 represents this other principle of a state of the art receiver for measuring frequencies of the signal received.
The measurement of phase shift between the signal received s(t) and the signal received delayed by τ s(t-τ) is performed by a mixer of SSB type 32 (single sideband) receiving through a mixing input 34 the signal received s(t) and through another mixing input the signal s(t-τ) delayed by τ. The frequency of the signal Fmeasured is deduced therefrom:
  Fmeasured  =      φ          2      ⁢              π        ·        τ            
The phase is measured to within 2π. To obtain a non-ambiguous measurement in a given frequency domain, the line length (delay τ) is limited. On the other hand, for a given phase error, the larger τ, the more precise the frequency measurement. To obtain the required frequency precision without ambiguity, it is necessary to use several delay lines simultaneously. The longest line gives the precision, the other line, or lines, make it possible to resolve the ambiguities.
The state of the art receivers implementing these solutions for measuring the frequency of the signal received comprise drawbacks notably:
significant volume and cost;
use analog functions which are therefore subject to drifting such as:                variations in the delays as a function of temperature,        Imperfections of the level measurements (spatial sampling solution);        imperfections of the phase shift measurements (auto-correlator with delay line solution).        
Consequently, it is necessary, in these state of the art solutions, to resort to adjustments and calibrations to compensate for this analog drifting and improve the precision and fidelity of the frequency measurements. This contributes, furthermore, to increasing the cost of these solutions and limiting the precision.
The state of the art solutions are not suitable for extracting the phase modulation of the signal. This is because a phase jump in the incident signal is visible only for a transient of duration corresponding to the propagation time in the delay line. This duration being very low, it is difficult to utilize the item of information.